The invention is a hypodermic needle assembly for the injection of local anesthetic, or other therapeutic fluid, into the tissue under pressure, particularly where the needle penetration is shallow.
At least three known shallow injections of local anesthetic used in dentistry are wonderfully effective when they work but can be difficult for the doctor to perform and uncertain in their results. They are known as 1) the periodontic ligament (PDL) injection where the gingival tissue meets each tooth across the thin band of ligament which ties all teeth to the bone; 2) the intrapulpal injection through an opening in the tooth enamel into the pulp chamber of the tooth hoping, of course, to find the root canal; and 3) the palatal or anterior middle superior alveolar (AMSA) injection in the shallow tissue of the roof of the mouth, believed to have been first discovered by the applicant, and dedicated to the public domain in 1996.
The earlier discovery by the applicant of apparatus and method by means of which many injections of local anesthetic can be performed substantially free of pain is disclosed in U.S. Pat. Nos. 4,747,824 and 5,180,371 and, using a different approach, in pending U.S. applications Ser. Nos. 09/122,915 and 09/394,958. These inventions all utilize precisely controlled flow rates for local anesthetic into the tissues over a wide range of pressures demanded by the equally wide range of tissue densities and distances which must be overcome in the body between the point of discharge of the anesthetic at the lumen of the implanted needle and the targeted nerves to be disabled to effect local numbing. Not infrequently, however, in shallow injections, the time for numbing to occur is prolonged or worse, never occurs, even though modern technology has made the procedure more bearable for the patient.
The present invention has for its purpose to improve measurably the success rate of local anesthetic injections, particularly when the hypodermic needle is implanted in shallow sites. In accordance with the invention, the shank of the needle is enveloped and enlarged by a sheath or lamination of elastomeric material with the interface between the sheath and the needle shank being such that at least the forward portion of the sheath can be slid backward when pressed from the forward end. The sliding motion is countered further back along the shank. Thus the sharp needle can cut its way into the tissue while the tissue surrounding the puncture hole engages the sheath to cause compression against the inherent elasticity. The sheath then forms a pressure seal around the needle shank where it enters the tissue.
When an injection is shallow, there is created a likely shunt path for leakage flow of anesthetic liquid under pressure out of the hole made by the needle. This leakage path is particularly vulnerable as the pressure on the anesthetic liquid is raised in an attempt to drive it through the interior tissues to the targeted nerve site. This leakage path is, however, sealed by the elastomeric sheath so that the pressure of local anesthetic is maintained over a wide range of injection pressures, while the injection apparatus continues to furnish its predetermined, controlled flow rate.
Elastomeric sleeves have been used for years, in a different way for a different purpose, in the practice of phlebotomy in which veins are tapped for obtaining blood samples. See U.S. Pat. Nos. 3,585,984 and 6,024,710 inter alia. A hollow needle is inserted in a vein so that blood flows naturally out of the back end to be collected in a specimen vial. The vial was evacuated and covered by a sealing membrane to boost the natural flow and to reduce the oxygen to which the sample is exposed. An elastomeric sleeve was added to the back end of the needle to act as a simple valve covering the discharge to keep blood from dripping out when no specimen vial is in place. The sleeve valve opens when pushed back by the puncturable sealing membrane covering the vial.
As such, the prior art is structurally and functionally different from the present invention in which the elastomeric sleeve is a pressure seal fitted on the front shank of a high pressure injection needle itself and which blocks the leakage flow path of local anesthetic discharged from the needle and flowing backward along the shank of the needle in the hole made in the tissue by the needle. The critical pressure seal is effected on the surface of the very tissue penetrated by the needle.